Code
P1200
SATURN
P — Powertrain
Injector Control Circuit
AI status
Completed
Completed
100%
Causes
- Open or shorted injector supply or driver circuit wiring
- Corroded, loose, or damaged injector connector pins
- Faulty fuel injector (coil short, open, intermittent)
- Bad injector driver in PCM (internal short/open)
- Loss of battery positive feed or ground to injector circuit
- Water/contaminant intrusion in connectors or harness damage (chewing, chafing)
Symptoms
- Malfunction Indicator Lamp (MIL) illuminated
- Rough idle or engine misfire on affected cylinder(s)
- Poor fuel economy and rough running under load
- Difficulty starting, stalling, or hesitation
- Diagnostic trouble codes for injector circuit(s) present
- Reduced engine performance if PCM enters limp mode
What to check
- Retrieve freeze frame and live data with a scan tool; identify which injector/cylinder is referenced
- Visually inspect injector harness, connectors, and pins for corrosion, bent pins, or damage
- Check for rodent/harness damage and any recent service that disturbed wiring
- With ignition ON (engine off) verify battery feed voltage at injector connector (should be ~battery voltage on the feed terminal)
- Measure injector coil resistance with an ohmmeter at the connector (compare to spec or other injectors)
- Backprobe connector while cranking/running to confirm driver pulse (use a noid light or oscilloscope)
Signal parameters
- Injector supply: switched battery voltage feed (approx. 12V with ignition ON)
- Injector driver: PCM typically grounds the injector to energize it (signal is a pulsed ground)
- Injector coil resistance: varies by design — low-impedance injectors commonly ~2–5 Ω, high-impedance ~12–16 Ω (compare to manufacturer spec and other injectors)
- Idle pulse width: typically a few milliseconds (commonly ~2–5 ms at idle; varies with load and engine speed)
- Waveform: square pulse to ground from PCM; use an oscilloscope to verify clean switching and expected pulse widths and duty cycle
Diagnostic algorithm
- Connect a suitable scan tool and record stored freeze-frame data and injector-related PIDs. Note which injector or circuit the PCM flagged.
- Visually inspect the injector harness, connectors, and PCM connector for corrosion, bent pins, heat damage, or water intrusion. Repair any obvious damage.
- With ignition OFF disconnect the injector harness and measure injector coil resistance at the pins; compare with service spec or other injectors. Replace any injector with an out-of-spec reading.
- With ignition ON (engine off) backprobe the connector feed terminal to verify battery voltage present. If feed is missing, trace power feed and fuses.
- Install a noid light at the injector harness and crank engine to verify the PCM is commanding the injector. If no light, check driver output at the PCM.
- Backprobe the driver terminal while cranking/running with a multimeter or, preferably, an oscilloscope to confirm pulsed ground waveform. Look for erratic or missing pulses or abnormal voltage levels.
- Perform a wiggle test on wiring and connectors while observing live data or oscilloscope for intermittent faults.
- Swap the suspect injector with a known-good injector from another cylinder. If the code or misfire moves with the injector, replace the injector. If it stays with the same cylinder, suspect wiring or PCM driver.
- Check continuity from the injector connector to the PCM pin. Repair any shorts to ground or battery, open circuits, or high-resistance connections.
- If wiring and injectors test good and waveform is abnormal or absent at the PCM output, consider PCM driver failure; confirm with manufacturer diagnostic procedures before replacing ECM.
- After repairs, clear codes and test drive to verify the fault does not return; re-check live data and misfire counters.
Likely causes
- Damaged or corroded injector connector or wiring at that cylinder
- Faulty injector coil (open or shorted)
- Intermittent wiring open/short to battery or PCM ground
- PCM driver failure (less common)
Fault status
Status
Stored when the PCM detects an abnormal condition in an injector control circuit (open, short, or improper driver signal). The fault may be current or intermittent; further testing required to locate wiring, injector, or PCM issue.
Repair difficulty: Medium
Diagnostic time: 1.5-3.0 hours
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